The present invention relates to communication through a borehole, such as between a surface and a downhole location in such a borehole, by transmitting acoustic waves lengthwise in such a borehole with liquid therein being the transmission medium. It more specifically relates to a transducer for converting between electrical power and acoustical power in such a borehole liquid, which transducer is not dependent upon movement of the liquid in order to provide communication.
One of the more difficult problems associated with any borehole is to communicate intelligence between one or more locations down a borehole and the surface, or 18 between downhole locations themselves. For example, communication is desired by the oil industry to retrieve, at the surface, data generated downhole during drilling operations, including during quiescent periods interspersing actual drilling procedures or while tripping; during completion operations such as perforating, fracturing, and drill stem or well testing; and during production operations such as reservoir evaluation testing, pressure and temperature monitoring. Communication is also desired in such industry to transmit intelligence from a surface to downhole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable downhole communication is particularly important when data (intelligence) is to be communicated, i.e., when more than a simple trigger signal or the like has to be communicated. This intelligence often is in the form of an encoded digital signal.
One approach which has been widely considered for borehole communication is to use a direct wire connection between the surface and the downhole location(s). Communication then can be via electrical signal through the wire. While much effort has been expended toward "wireline" communication, this approach has not been adopted commercially because it has been found to be quite costly and unreliable. For example, one difficulty with this approach is that since the wire is often laid via numerous lengths of a drill stem or production tubing, it is not unusual for there to be a break or poor wire connection formed at the time the wire assembly is first installed. While it has been proposed (see U.S. Pat. No. 4,215,426) to avoid the problems associated with direct electrical coupling of drill stems by providing inductive coupling for the communication link at such location, inductive coupling has as a problem, among others, major signal loss at every coupling. It also relies on installation of special and complex drill string arrangements.
Another borehole communication technique that has been explored is the transmission of acoustic waves. Such physical waves need a transmission medium that will propagate the same. It will be recognized that matters such as variations in earth strata, density make-up, etc., render the earth completely inappropriate for an acoustic communication transmission medium. Because of these known problems, those in the art generally have confined themselves to exploring acoustic communication through borehole related media.
Much effort has been expended toward developing an appropriate acoustic communication system in which the borehole drill stem or production tubing itself, acts as the transmission medium. A major problem associated with such arrangements is caused by the fact that the configurations of drill stems or production tubing generally vary significantly lengthwise. These variations typically are different in each hole. Moreover, a configuration in a particular hole may vary over time because, for example, of the addition of tubing, etc. The result is that there is no general usage system relying on drill stem or production tubing transmission that has gained meaningful market acceptance.
Efforts have also been made to utilize liquid within a borehole as the acoustic transmission medium. At first blush, one would think that use of a liquid as the transmission medium in a borehole would be a relatively simple approach, in view of the wide usage and significant developments that have been made for communication and sonar systems relying on acoustic transmission within the ocean. In fact, U.S. Pat. No. 3,233,674 is an example of a patent which simply mentions such communication without taking into consideration the fact that those skilled in the art may not be able to implement the same.
Acoustic transmission of a liquid within a borehole is considerably different than acoustic transmission within an open ocean because of the problems associated with the boundaries between the liquid and its confining structures in a borehole. Criteria relating to these problems are of paramount importance. However, because of the attractiveness of the concept of acoustic transmission in a liquid independent of movement thereof, a system was proposed in U.S. Pat. No. 3,964,556 utilizing pressure changes in a non-moving liquid to communicate. Such system has not been found practical, however, since it is not a self-contained system and some movement of the liquid has been found necessary to transmit pressure changes.
In light of the above, meaningful communication of intelligence via borehole liquids has been limited to systems which rely flow of the liquid to carry on acoustic modulation from a transmission point to a receiver. This approach is generally referred to in the art as MWD (measure while drilling). Developments relating to it have been limited to communication during the drilling phase in the life of a borehole, principally since it is only during drilling that one can be assured of fluid which can be modulated flowing between the drilling location and the surface. Most MWD systems are also constrained because of the drilling operation itself. For example, it is not unusual that the drilling operation must be stopped during communication to avoid the noise associated with such drilling. Moreover, communication during tripping is impossible.
In spite of the problems with MWD communication, much research has been done on the same in view of the desirability of good borehole communication. The result has been an extensive number of patents relating to MWD, many of which are directed to proposed solutions to the various problems that have been encountered. U.S. Pat. No. 4,215,426 describes an arrangement in which power (rather than communication) is transmitted downhole through fluid modulation akin to MWD communication, a portion of which power is drained off at various locations downhole to power repeaters in a wireline communication transmission system.
The development of communication using acoustic waves propagating through non-flowing fluids in a borehole has been impeded by lack of a suitable transducer. To be practical for a borehole application, such a transducer has to fit in a pressure barrel with an outer diameter of no more than 1.25 inches, operate at temperatures up to 150.degree. C. and pressures up to 1000 bar, and survive the working environment of handling and running in a well. Such a transducer would also have to take into consideration the significant differences between communication in a non-constrained fluid environment, such as in the ocean, and a confined fluid arrangement, such as in a borehole.